Sometimes we need to assume certain number of terms in Arithmetic Progression. The following ways are generally used for the selection of terms in an arithmetic progression.

(i) If the sum of three terms in Arithmetic Progression be given, assume the numbers as a - d, a and a + d. Here common difference is d.

(ii) If the sum of four terms in Arithmetic Progression be given, assume the numbers as a - 3d, a - d, a + d and a + 3d.

(iii) If the sum of five terms in Arithmetic Progression be given, assume the numbers as a - 2d, a - d, a, a + d and a + 2d. Here common difference is 2d.

(iv) If the sum of six terms in Arithmetic Progression be given, assume the numbers as a - 5d, a - 3d, a - d, a + d, a + 3d and a + 5d. Here common difference is 2d.

**Note:** From the
above explanation we understand that in case of an odd number of terms, the
middle term is ‘a’ and the common difference is ‘d’.

Again, in case of an even number of terms the middle terms are a - d, a + d and the common difference is 2d.

Solved examples to observe how to use the selection of terms in an arithmetic progression

**1.** The sum of three numbers in Arithmetic Progression is 12 and
the sum of their square is 56. Find the numbers.

**Solution:**

Let us assume that the three numbers in Arithmetic Progression be a - d, a and a + d.

According to the problem,

Sum = 12 and
⇒ a - d + a + a + d = 12 ⇒ 3a = 12 ⇒ a = 4 |
Sum of the squares = 56
(a - d)\(^{2}\) + a\(^{2}\) + (a + d)\(^{2}\) = 56 ⇒ a\(^{2}\) - 2ad + d\(^{2}\) + a\(^{2}\) + a\(^{2}\) + 2ad + d\(^{2}\) = 56 ⇒ 3a\(^{2}\) + 2d\(^{2}\) = 56 ⇒ 3 × (4)\(^{2}\) + 2d\(^{2}\) = 56 ⇒ 3 × 16 + 2d\(^{2}\) = 56 ⇒ 48 + 2d\(^{2}\) = 56 ⇒ 2d\(^{2}\) = 56 - 48 ⇒ 2d\(^{2}\) = 8 ⇒ d\(^{2}\) = 4 ⇒ d = ± 2 |

If d = 3, the numbers are 4 – 2, 4, 4 + 2 i.e., 2, 4, 6

If d = -3, the numbers are 4 + 2, 4, 4 - 2 i.e., 6, 4, 2

Therefore, the required numbers are 2, 4, 6 or 6, 4, 2.

**2.** The sum of four numbers in Arithmetic Progression is 20 and the sum of their square is 120. Find the numbers.

**Solution:**

Let us assume that the four numbers in Arithmetic Progression be a - 3d, a - d, a + d and a + 3d.

According to the problem,

Sum = 20
⇒ a - 3d + a - d + a + d + a + 3d = 20 ⇒ 4a = 20 ⇒ a = 5 |
and |
Sum of the squares = 120
⇒ (a - 3d)\(^{2}\) + (a - d)\(^{2}\) + (a + d)\(^{2}\) + (a + 3d)\(^{2}\) = 120 ⇒ a\(^{2}\) - 6ad + 9d\(^{2}\) + a\(^{2}\) - 2ad + d\(^{2}\) + a\(^{2}\) + 2ad + d\(^{2}\) + a\(^{2}\) + 6ad + 9d\(^{2}\) = 120 ⇒ 4a\(^{2}\) + 20d\(^{2}\) = 120 ⇒ 4 × (5)\(^{2}\) + 20d\(^{2}\) = 120 ⇒ 4 × 25 + 20d\(^{2}\) = 120 ⇒ 100 + 20d\(^{2}\) = 120 ⇒ 20d\(^{2}\) = 120 - 100 20d\(^{2}\) = 20 ⇒ d\(^{2}\) = 1 ⇒ d = ± 1 |

If d = 1, the numbers are 5 - 3, 5 - 1, 5 + 1, 5 + 3 i.e., 2, 4, 6, 8

If d = -1, the numbers are 5 + 3, 5 + 1, 5 - 1, 5 - 3 i.e., 8, 6, 4, 2

Therefore, the required numbers are 2, 4, 6, 8 or 8, 6, 4, 2.

**3.** The sum of three numbers in Arithmetic Progression is -3 and
their product is 8. Find the numbers.

**Solution:**

Let us assume that the three numbers in Arithmetic Progression be a - d, a and a + d.

According to the problem,

Sum = -3 and
⇒ a - d + a + a + d = -3 ⇒ 3a = -3 ⇒ a = -1 |
Product = 8
⇒ (a - d) (a) (a + d) = 8 ⇒ (-1)[(-1)\(^{2}\) - d\(^{2}\)] = 8 ⇒ -1(1 - d\(^{2}\)) = 8 ⇒ -1 + d\(^{2}\) = 8 ⇒ d\(^{2}\) = 8 + 1 ⇒ d\(^{2}\) = 9 ⇒ d = ± 3 |

If d = 3, the numbers are -1 - 3, -1, -1 + 3 i.e., -4, -1, 2

If d = -3, the numbers are -1 + 3, -1, -1 - 3 i.e., 2, -1, -4

Therefore, the required numbers are -4, -1, 2 or 2, -1, -4.

**●** **Arithmetic Progression**

**Definition of Arithmetic Progression****General Form of an Arithmetic Progress****Arithmetic Mean****Sum of the First n Terms of an Arithmetic Progression****Sum of the Cubes of First n Natural Numbers****Sum of First n Natural Numbers****Sum of the Squares of First n Natural Numbers****Properties of Arithmetic Progression****Selection of Terms in an Arithmetic Progression****Arithmetic Progression Formulae****Problems on Arithmetic Progression****Problems on Sum of 'n' Terms of Arithmetic Progression**

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